National Water Account 2015

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Water Accounting Statements

Water Assets and Water Liabilities

Changes in Water Assets and Water Liabilities

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Notes

Accounting policies

Supporting information

Quantification approach

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References

Accountability statement

Canberra: Quantification approaches

• Summary of quantification approaches
• Detail of quantification approaches

Summary of quantification approaches

Table N14 outlines the quantification approaches used to derive the item volumes for the Canberra region. For a more detailed description of the quantification approach, click on the relevant item name in the table.

Table N14  Quantification approaches used to derive item volumes

Detail of quantification approaches

 Water storage product data

Storages

Storage volume at the start and end of the year was calculated using water level data (metres above Australian Height Datum) collected at each storage. Capacity tables established for each storage were used to convert the height measurement to a volume.

The volume of individual storages was aggregated to present the total volume as detailed in the Surface water note. The uncertainty range for these volumes is +/–5%.

The assumptions made were as follows:

• Storage–volume curves represent specifically surveyed parts of the storage and may not reflect the storage–volume relationship across the entire storage.
• Storages are subject to sedimentation and other physical changes over time that in turn affect the accuracy of the storage–volume curves.

Lakes and wetlands

The volume of water in lakes and wetlands is based on both measured and estimated data. The volume of water in Lake Burley Griffin at the start and end of the year was calculated using water level data (metres above Australian Height Datum) collected at the lake. Rating tables established for the lake were used to convert the height measurement to a volume.

The volume of water in Lake Ginninderra and Lake Tuggeranong was estimated based on the known capacity of the lakes, that is, the lakes were assumed to be full at 30 June 2015.

The assumptions made were as follows:

• Water levels in Lake Ginninderra and Lake Tuggeranong are generally managed within 200 mm of full supply level throughout the year. Therefore, the estimated storage volumes of these lakes are considered to be only slightly overestimated.
• At present, abstraction from Lake Ginninderra and Lake Tuggeranong is limited to volumes that would have negligible effect on the volume of the lakes (<50 ML).
• The capacity of Lake Burley Griffin is based on survey data collected at the time of construction and fill in 1964.

Service reservoir data

Supply system

The storage volume of the urban water supply system is comprises two components: the distribution pipe network and service reservoirs.

The volume in the pipe network was calculated based on the pipe lengths and nominal internal pipe diameters as recorded in the Geographic Information System (GIS) database. This includes both nonpotable and potable water pipes. It was assumed that all the pipes were full on 30 June 2015.

Asset database

Recycled water

The volume in pipes was calculated using Icon Water's Esri GIS asset database, and assumes all pipes in the network are full. Two storage tanks are used for recycled water, the LMWQCC nonpotable water tank and the Lower Russell tank. These were assumed to have a combined storage of approximately 1 ML to give a total system storage of 2 ML (rounded to nearest ML).

Water Sharing Plan for the Murrumbidgee Unregulated and Alluvial Water Sources

Allocation: individual users

The Water Sharing Plan for the Murrumbidgee Unregulated and Alluvial Water Sources 2012 details the rules for surface water and groundwater allocations for access licences with share components.

Allocation remaining

The remaining surface water and groundwater allocation corresponds to the volume of water allocation that can be carried over between water years. The water allocation remaining at 30 June 2015 is calculated as shown in Table N15.

Adjustment and forfeitures: individual users

Carry-over of unused allocation is allowed in Water Sharing Plan for the Murrumbidgee Unregulated and Alluvial Water Sources 2012. While carryover of unused surface water allocation is allowed for access licence holders, due to limited data availability the carryover volume could not be quantified. Therefore, for the purposes of the National Water Account, the closing balance of remaining allocation is zero.

Water resource information database

Allocation: urban system

Water access entitlement volumes were obtained from the ACT water resource information database. The total annual announced allocation for 2014–15 year was considered to be 100% of the urban water licence entitlement volume for Icon Water. There is no distinction between regulated and unregulated flows in Icon's urban water access entitlement. Water to service this licence was diverted from both regulated and unregulated sources.

Adjustment and forfeiture: urban system

Allocated water that had not been diverted was considered as forfeited.

Non-allocated diversion: individual users

Surface water non-allocated diversion includes diversions of unregulated flows under multi-use licences for nonpotable water within the ACT. It includes licensed diversions within urban area for unspecified purposes and licensed diversions for all purposes outside the urban area.

Individual licensees submit monthly meter readings to the ACT Environment Protection Authority (EPA) on an annual basis. Where a licensee has not provided meter data, the EPA estimates usage based on historical usage data. The uncertainty range for these volumes is +/–5%. Uncertainty can not be quantified for estimated usage where meters were not read.

Non-allocated extraction: individual users

For non-allocated groundwater extraction in the ACT, individual licensees submit meter readings for each month to the ACT EPA once per year. Where a licensee has not provided meter data, the EPA may assume use as 100% of entitlement or it may estimate the usage based on historical usage data. The uncertainty range for these metered volumes is +/–5%. Uncertainty can not be quantified for estimated usage where meters were not read.

Stream monitoring data

Inter-region: inflow / outflow

The river outflow from the region represents the volume of water that flows out of the Canberra region from the Murrumbidgee River. The volume is recorded at Murrumbidgee River at Hall's Crossing (gauging station 410777) (Figure N14).

Figure N14 Gauging stations used to calculate river flows to and from the region

The limitations associated with this approach are:

• The level of uncertainty of these gauging stations is estimated at +/– 20 % during low to medium flows and the uncertainty during high flows is ungraded. This is based on four manual physical flow gaugings performed per year. At these sites the water surface level is measured constantly by on-site equipment. This water level is used to estimate a flow rate, based on a rating curve produced by physical flow gauging in as many flow conditions as possible.

• As both Angle Crossing and Halls Crossing sites are located not far away from the Canberra region boundary it was assumed that minimal runoff was generated between the catchment boundary and the monitoring site.

• There is some uncertainty in the flow rates. The river flows have not been gauged under all flow conditions, and the river channel can change from time to time due to deposition and movement of river sediments. It impacts the cross-sectional area of the channel and changes the velocity of the water.

Gridded climate data and AWRA-L model

Precipitation and evaporation

Monthly precipitation grids for the region were produced using daily data from approximately 6,500 rain gauge stations across Australia and interpolated to a 0.05 degree (5 km) national grid (Jones et al. 2009).

Potential evapotranspiration across the region was estimated using the Australian Water Resources Assessment system Landscape model (AWRA-L) version 5.0 (Viney et al. 2015). The AWRA-L model uses the Penman method to produce the potential evapotranspiration. Daily AWRA-L potential evapotranspiration grids for the region were produced based on daily gridded climate data (including precipitation, solar radiance, and temperature) available on a 0.05 degree (approximately 5 km) national grid (Jones et al. 2009).

The precipitation and evapotranspiration at each waterbody were estimated from the proportionally weighted average of grid cells that intersected each water feature. The volume was then estimated by multiplying by the surface area of each waterbody. The average monthly surface area of the major storages was calculated from daily storage levels and capacity tables. In the Canberra region, the surface area of the four storages was calculated dynamically and the surface area of the three urban lakes was a static value produced from the Australian Hydrological Geospatial Fabric (AHGF).

The limitations associated with this approach are:

• The precipitation and AWRA-L potential evapotranspiration estimates were subject to approximations associated with interpolating the observation point data to a national grid as detailed in Jones et al. (2009).
• The dynamic storage surface areas calculated from the levels and storage rating tables represent a monthly average and therefore will not capture changes that occur on a shorter temporal scale.
• The use of the static default AHGF surface area is an approximation only. 
• The total surface area of the surface water store within the Canberra region included only the reservoirs and urban lakes. River channels were not included.

Runoff

Runoff to surface water in the Canberra region was based on streamflow estimates from the AWRA-L version 5.0 model outputs. Using gridded climate data for the Canberra region (including precipitation, temperature, and solar radiation data), AWRA-L was used to estimate the runoff depth at each grid cell within the region. Only runoff from the landscape was considered; therefore, the surface areas of the major storages were excluded from the analysis.

Runoff from the landscape is divided into three components: runoff into the major storages; runoff into farm dams; and remaining runoff in the region. Runoff into farm dams was not considered in this item.

The runoff from the catchment contributing to major storages and the remaining runoff within the region were separately calculated.

Figure N15 Runoff area to storages and modelled area within the Canberra region

The average runoff depth from the landscape into the connected surface water store was determined as the weighted mean of the relevant grid cells within the region boundary. Points were weighted based upon the area they represented within the reporting region to remove edge effects (where the area represented is not wholly within the reporting region) and the effect of changing area represented with changing latitude. Runoff depth was converted to a runoff volume by multiplying runoff depth by the total area of the region (excluding storages).

The limitations associated with this approach are:

• The runoff estimates were subject to the assumptions of the AWRA-L version 5.0 model detailed by Viney et al. (2015).
• The estimated runoff corresponds to the runoff expected from an unimpaired catchment. The impairment on runoff from farm dams was estimated using a farm dam water balance model (farm dam algorithm written by the Bureau of Meteorology). Where this was applied, the runoff estimates inherited the approximations, assumptions, caveats, and the parameters used for total grid point area.

Metered and estimated data provided by water authorities

Allocated diversion: urban system

The allocated diversion: surface water volume is calculated from the volume of surface water diverted (metered at the source). The uncertainty range for these volumes is +/–5%.

Where metered inflows to water treatment plants were not available, these volumes were assumed to equal the metered outflow volume (i.e., no water losses occurred during the treatment process). 

Non-allocated diversion: urban system

The non-allocated diversion: surface water volume is calculated from the volume of surface water diverted (metered at the source) and inflow to water treatment plants.

This is the metered volume of potable water discharged from the urban water supply system to Googong Reservoir, to take advantage of storage capacity. This volume is not counted as part of the volume diverted under Icon Water’s entitlement. The estimated uncertainty range for these volumes is +/–5%.

Wastewater collected

The volume of wastewater collected is estimated using the aggregated metered inflow to wastewater treatment plants within the region:

• minus any recirculation such as treated wastewater volume that was reported as discharge to avoid double counting
• plus any reported wastewater losses or egress from the system before the metering point measuring inflow to the treatment plants (e.g., through emergency relief structure).

The assumptions made were as follows:

• Given wastewater volumes were typically measured at the treatment plants (and not at customer connections), the collected wastewater volumes included any variation due to (a) ingress of stormwater; (b) infiltration of groundwater; (c) unreported wastewater overflows to stormwater; and (d) exfiltration of wastewater to groundwater. 
• Where inflow meter readings were not available, outflow meter readings were used, which could underestimate the volume as they assumed no losses during wastewater treatment.
• The volume of wastewater collected does not include wastewater for individual or community wastewater management systems.

Uncertainty information:

• The uncertainty of the volume of wastewater collected is estimated to be of the order of +/–20%.
• The uncertainty in the estimated volume of storm water and / or groundwater ingress is in the order of +/- 50%.

Supply system delivery: urban users

The Supply system delivery: urban users volume includes urban consumption of potable and non-potable water and is derived from:

• customer meters
• billing meters, and
• estimated non-revenue water volumes.

Urban consumption consists of residential, commercial, industrial, municipal use, and small scale agriculture / irrigation uses. The volume delivered to non-urban users (i.e., supply to irrigation schemes and the environment) is not included in the reported volume. The estimated uncertainty range for these volumes is +/–10%.

Supply system discharge: surface water

Evaporation: urban system

The volume of evaporation from the urban water system was:

• calculated using the Bureau climate data

• calculated using a water balance approach through available inflow and outflow metering data; and/or
• the metered treated wastewater volume disposed to evaporation lagoons.

Evaporation losses are only reported for the wastewater system.

Leakage: groundwater

The leakage: groundwater volume is assumed to be the non-revenue water associated with real losses: specifically due to background pipe leakage from the urban water supply system. Real losses related to pipe bursts are reported in Leakage: landscape.

Non-revenue water is estimated using:

• the difference based on a water balance between metered water produced and revenue water (metered/estimated customer consumption charges); and/or
• modelling software of network real losses (leakages and bursts) and apparent losses (unauthorised/authorised unbilled use); and
• time to repair leaks.

The real losses component of Non-revenue water reported as Leakage: groundwater is based on both avoidable and unavoidable losses (including pipe network background leaks, pipe leaks and bursts, tank and service reservoir leakage, and overflows) and is calculated using the following equation:

Real losses = Non-revenue water – (Apparent losses + Unmetered authorised consumption).

The volume of apparent losses comprises two components: unauthorised consumption (e.g., water theft); and customer meter under-registration (e.g., meter inaccuracies).

The following assumptions were made:

• Leakage in the wastewater system is not reported and therefore the total leakage to groundwater is likely to be underestimated.
• Where non-revenue water real losses are reported as a combined volume for pipe bursts and background leakage, with no breakdown, this was reported in Leakage: groundwater, which may overestimate the volume.

Other supply system decreases

The other supply system decreases volume is the remaining non-revenue water from the urban water supply system (if not reported in 'Leakage to landscape' and 'Leakage to groudnwater' respectively)

 Remaining non-revenue water was estimated using:

• the difference based on a water balance between metered water sourced and supplied to customers; and/or
• the difference between metered supply into the urban water supply system and metered volume of water consumed (revenue water) and subtracting real losses; and/or
• modelling software of network real losses (leakages and busts) and apparent losses (unauthorised/authorised unbilled use); and/or
• time to repair leaks; and/or
• difference between inlet meter and outlet meter of water treatment plants for treatment losses.

The following assumptions were made:

• Non-potable water losses were not reported and losses in the system were likely to be underestimated.

Recycled water delivery: urban users

The recycled water delivery: urban users is derived from:

• customer meters
• billing meters onsite re-use water meters.

The volume excludes recycled water re-circulated within the wastewater treatment process.

Urban consumption consists of residential, commercial, industrial, municipal, onsite (water and wastewater treatment plant) use, and small scale agriculture/irrigation uses.

The estimated uncertainty range for these volumes is +/–5%.